This invention relates to resistance spot welders and more particularly to controlling the electrical current by provision of a current feedback control to assure the desired current is applied independent of variations in the welding equipment and line voltage.
The manufacture of jet engine and other parts requires hundreds of spot welding operations. The quality of the weld is largely dependent on the power into the spot weld, where power is the product of current squared times resistance. Since the process is influenced by the square of the current, a small perturbation in current can result in unacceptable welds. A manufacturing plant may have a dozen or more spot welding machines in production. Due to variations in these machines, welding schedules must be manually adjusted to accommodate these variations. The welding is performed based on a weld schedule, which sets the current and duration. Presently the welding equipment runs in open loop mode, that is, a current is commanded but nothing verifies it is actually applied.
The invention may be used in a system that has an adaptive control for controlling the spot welding process in real time and is capable of dealing with the numerous process parameter variations and non-linearities present in this welding process. The adaptive control adjusts the heat input to the process to compensate for electrode wear, oxidized surfaces, fit up variations and machine variations. The controller in copending allowed application Ser. No. 185,599, filed Apr. 25, 1988, "Real Time Adaptive Control for Resistance Spot Welding Process", K. B. Haefner et al, now abandoned, and continuation application Ser. No. 299,102 filed Jan. 19, 1989, embodies several unique features which allow accurate control of the process within the tight power and time constraints necessary to produce a good weld.